Editing Single system image (section)

==Features of SSI clustering systems==

Different SSI systems may, depending on their intended usage, provide some subset of these features.

===Process migration===
{{main|Process migration}}
Many SSI systems provide [[process migration]].<ref name=Smith1988>{{citation
 | last = Smith | first =  Jonathan M.
 | year = 1988
 | title = A survey of process migration mechanisms
 | journal = ACM SIGOPS Operating Systems Review
 | volume = 22
 | issue =  3
 | pages = 28–40
 | doi = 10.1145/47671.47673
 | url = http://www.cis.upenn.edu/~jms/svy-pm.pdf
| citeseerx =  10.1.1.127.8095
 | s2cid = 6611633
 }}</ref>
Processes may start on one [[Node (networking)|node]] and be moved to another node, possibly for [[Load balancing (computing)|resource balancing]] or administrative reasons.<ref group="note">for example it may be necessary to move long running processes off a node that is to be closed down for maintenance</ref>  As processes are moved from one node to another, other associated resources (for example [[Inter-process communication|IPC]] resources) may be moved with them.

===Process checkpointing===
Some SSI systems allow [[Application checkpointing|checkpointing]] of running processes, allowing their current state to be saved and reloaded at a later date.<ref group="note">Checkpointing is particularly useful in clusters used for [[high-performance computing]], avoiding lost work in case of a cluster or node restart.</ref>
Checkpointing can be seen as related to migration, as migrating a process from one node to another can be implemented by first checkpointing the process, then restarting it on another node.  Alternatively checkpointing can be considered as ''migration to disk''.

===Single process space===
Some SSI systems provide the illusion that all processes are running on the same machine - the process management tools (e.g. "ps", "kill" on [[Unix]] like systems) operate on all processes in the cluster.

===Single root===
Most SSI systems provide a single view of the file system. This may be achieved by a simple [[Network File System (protocol)|NFS]] server, shared disk devices or even file replication.

The advantage of a single root view is that processes may be run on any available node and access needed files with no special precautions.  If the cluster implements process migration a single root view enables direct accesses to the files from the node where the process is currently running.

Some SSI systems provide a way of "breaking the illusion", having some node-specific files even in a single root. [[Hewlett-Packard|HP]] [[TruCluster]] provides a "context dependent symbolic link" (CDSL) which points to different files depending on the node that accesses it.  [[Hewlett-Packard|HP]] [[VMScluster]] provides a search list logical name with node specific files occluding cluster shared files where necessary. This capability may be necessary to deal with ''heterogeneous'' clusters, where not all nodes have the same configuration. In more complex configurations such as multiple nodes of multiple architectures over multiple sites, several local disks may combine to form the logical single root.

===Single I/O space===
Some SSI systems allow all nodes to access the I/O devices (e.g. tapes, disks, serial lines and so on) of other nodes.  There may be some restrictions on the kinds of accesses allowed (For example, [[OpenSSI]] can't mount disk devices from one node on another node).

===Single IPC space===
Some SSI systems allow processes on different nodes to communicate using [[inter-process communication]]s mechanisms as if they were running on the same machine.  On some SSI systems this can even include [[Shared memory (interprocess communication)|shared memory]] (can be emulated in software with [[distributed shared memory]]).

In most cases inter-node IPC will be slower than IPC on the same machine, possibly drastically slower for shared memory.  Some SSI clusters include special hardware to reduce this slowdown.

===Cluster IP address===
Some SSI systems provide a "[[cluster IP]] address", a single address visible from outside the cluster that can be used to contact the cluster as if it were one machine.  This can be used for load balancing inbound calls to the cluster, directing them to lightly loaded nodes, or for redundancy, moving the cluster address from one machine to another as nodes join or leave the cluster.<ref group=note>"leaving a cluster" is often a euphemism for crashing</ref>